What kind of culture was used? If it was like MM100 or Flora Danica, types containing ssp diacetylactis or Leuc. mesenteriodes that also product gas, it could have been from them. But I'd expect perhaps more and more evenly distributed holes in that case? Just the other day I forgot about a Baby Swiss I was brining, left it in for 50% again longer than intended...will see what fate I suffer.

I may have misunderstood/ memory error, but I think I read that too much salt kills PS. and the holes mostly in the center seem to back that up. It doesn't taste particularly salty, but my better half uses so much salt I have been numbed to its taste.